Cholera is caused by the bacterium Vibrio cholerae. Most str…

Cholera is caused by the bacterium Vibrio cholerae. Most strains of V. cholerae are harmless, but those that produce ctx toxin cause cholera. Researchers performed experiments to determine how the highly pathogenic strain of V. cholerae acquired the ctx toxin gene. Experimental procedure: Step 1:  The pathogenic strain of V. cholerae bacteria were passed through a very fine filter (0.2µm). They found that the bacteria-free fluid left behind, the filtrate, could still transfer the ctx toxin gene to a non-pathogenic strain of V. cholerae cells.  Step 2: The filtrate was then treated with DNase, an enzyme that degrades DNA that is not surrounded by protein. The DNase-treated filtrate was still able to transfer the ctx toxin gene to the previously non-pathogenic strain.  These results proved that the ctx gene was transferred from the highly pathogenic strain of V. cholerae to the non-pathogenic strain by which of the following methods?  (HINT:  Consider your responses to the previous question to help you determine this answer.)